By DONALD G. McNEIL Jr.

Published: July 20, 2010

The battle to eliminate polio, which has been more than 99 percent successful and has hovered on the verge of victory for a decade, has sustained new setbacks.

There has been an outbreak of more than 300 cases in Tajikistan this year. (Tajikistan is just north of Afghanistan, and fighting on the Afghan-Pakistan border between the United States and the Taliban has let the disease surge in both countries.) Cases in the two permanent hot spots, Nigeria and India, are down but not eliminated, and the two continue to seed outbreaks in neighboring countries.

But the ''biggest bump in the road,'' Dr. John F. Modlin, a polio expert at Dartmouth Medical School wrote in a recent article in The New England Journal of Medicine, has been the recent emergence of polio cases caused by live vaccine viruses that mutated until they, too, were dangerous enough to cause paralysis. The problem was first discovered in the Dominican Republic and Haiti a decade ago; that outbreak was quelled. But since last year, it has happened in Nigeria, Congo, Somalia, India and Ethiopia. Case numbers are tiny, but the idea of vaccine leading to disease could scare people away from being vaccinated.

The vaccine drops themselves are safe, but the live virus they contain then circulates, protecting more people -- unless dangerous mutations occur. Because killed vaccines are expensive to make and to inject, the only practical solution for now is more aggressive oral vaccination, experts say.